DC-DC power converters implementing synchronous rectification use power MOSFET devices as the high-side power switch and the low-side power switch where the power switches operate to regulate the delivery of current to a load. In operation, both power switches are off before one is being turned on. During the time when both power switches are turned off, the body diode of the low-side MOSFET device conducts the load current. Because the body diode of a MOSFET device has a forward voltage of about 0.7 volt, conduction through the body diode results in significant conduction loss which degrades the efficiency of the power converter. Furthermore, the body diode of the MOSFET device has a high reverse-recovery charge, resulting in additional efficiency loss.
To improve the conversion efficiency of the power converter, a Schottky diode is often added in parallel with the MOSFET body diode, as shown in FIG. 1. When the power switches of the power converter are implemented using N-type MOSFET devices (or NMOS transistors), the NMOS transistor M1 has a body diode D1 formed by the P-type body region as the anode and the N-type drain region as the cathode. To improve conversion efficiency, a Schottky diode SD1 is connected in parallel with the body diode D1. The anode of the Schottky diode SD1 is electrically connected to the source terminal of the NMOS transistor M1 or the anode of the body diode D1. The cathode of the Schottky diode SD1 is electrically connected to the drain terminal of the NMOS transistor M1 or the cathode of the body diode D1. The Schottky diode SD1 has a lower forward bias voltage (e.g. 0.3V) than that of the body diode D1 and thus reduces the forward voltage drop as well as improves recovery time.
More specifically, a Schottky diode is a semiconductor device formed by a metal contacting a semiconductor layer. The junction between the metal and the semiconductor layer forms a rectifying junction with improved diode switching capability as compared to p-n junction diodes formed entirely in a semiconductor layer. Schottky diodes thus have lower turn-on voltages and faster switching speeds as compared to p-n junction diodes.
While it is desirable to connect a Schottky diode with a MOSFET device, integrating a Schottky diode with MOSFET devices increases the die size and the cost of the power converter.
FIG. 2 is a cross-sectional view of a conventional double-diffused MOS (DMOS) transistor which can be used as the power transistor in a power converter. DMOS transistors can be formed as vertical devices (vertical DMOS or VDMOS) or lateral devices (lateral DMOS or LDMOS). In the present example, a vertical DMOS transistor is shown. Referring to FIG. 2, an N-type vertical DMOS transistor 10 is formed on an N+ substrate 12 and an N-type epitaxial layer 14. The DMOS transistor 10 includes a gate electrode formed by a polysilicon layer 22 and insulated from the epitaxial layer 14 by a thin gate dielectric layer 20. The DMOS transistor 10 further includes a P-type body region 16 formed in the N-type epitaxial layer 14 and N+ source regions 18 formed in the P-body region 16. A source electrode is formed using a metal layer 26, formed above an insulating layer 24, connecting to both the N+ source region 18 and the P-body region 16. The N-type epitaxial layer 14 and the N+ substrate 12 form the drain of the DMOS transistor 10. A drain electrode is formed using a metal layer 28 formed on the backside of N+ substrate 12. The body region under the gate electrode between the N+ source region 18 and the N-epitaxial layer 14 form the channel region of the DMOS transistor. When the DMOS transistor 10 is turned on, the forward current flows vertically from the drain electrode through the N+ substrate and the N-type epitaxial layer through the channel to the source electrode.
The vertical DMOS transistor 10 includes a body diode formed by the P-body region 16 as the anode and the N-epitaxial layer 14 as the cathode. When the gate voltage is held above threshold, the P-body layer inverts and current can flow from drain to source when drain is positive as shown by the arrows. When the gate of the transistor is held at zero volts, the transistor is turned off and there will be no current even when the drain is at a positive voltage. However, if the drain is biased to a negative voltage, the body diode between P-body region and the N-epitaxial layer will turn-on and current can flow from the source to the drain even when the DMOS device is turned off. When the DMOS transistor 10 is used as a low-side power switch in a power converter, conduction through the body diode results in significant conduction loss due to the high turn on voltage (e.g. 0.7V) and the high reverse-recovery charge.